1. Field of the Invention
The present invention relates generally to air conveyor systems and more particularly to flow control in air conveyor systems.
2. Description of the Prior Art
In general, air conveyance systems are accumulators where the conveyor fills with product and then empties out. Common applications of air conveyor systems include handling two piece aluminum cans and blow molded plastic bottles.
An air conveyance system provides motive force to the product being conveyed through momentum transfer from an air jet to the product. When the system is full of product, the air pressure must be set high enough to clear out the entire weight of all the product present. When the system is empty and single items are moving through the system, there is a tendency for the single objects to move too fast and either become damaged or jammed. The difference between accumulated and unaccumulated flow may be analogized to the difference between bumper-to-bumper rush hour traffic and single cars moving at the speed they choose.
A conventional two pressure design is shown in FIG. 1. In this design, a blower 50 pressurizes a plenum 54 which supplies air 56 to the jets 58 which impart momentum to the product. When the sensor tells the control system that the subject fan zone has become accumulated (i.e., backed up), then the automatic damper 62 opens, supplying more air 56 into the plenum 54. Inlet dampers, outlet dampers, and variable speed drives which change the motor's revolutions per minute (RPM) have been used as methods for changing plenum pressure.
A second conventional two pressure design is shown in FIG. 2. In this design the plenum is split by a divider 68. A blower 70 pressurizes the primary plenum 74, and multiple dampers 78 in parallel regulate the flow of air 80 into the secondary plenum 82. Louvers which form the air jets 86 that drive the product are connected to the secondary plenum 82. U.S. Pat. No. 5,222,840 is representative of this type of conventional design.
A disadvantage with the first approach is that the length of the control zone is the same as the blower zone. Once the control zone becomes too long then the advantage of the two pressure system is lost. The sensor which switches the zone to high pressure must be located so far from the upstream end of the zone that the product has a chance to gain terminal speed before hitting the back end of the accumulated pack--thereby causing damage to the product and to the pack.
Moreover, the use of smaller blowers and a shorter blower zone in order to shorten the control zone increases both the cost of the equipment and the cost to install it. Lastly, a disadvantage with the second approach is the sheet metal fabrication cost to divide the two plenums.